This invention relates, in general, to a DC feedback control circuit and more particularly, though not exclusively, to such a circuit used for compensating for a DC level shift in a circuit using a resistor as a current source, for example a high frequency amplifier.
In integrated circuits, a DC voltage applied to a particular component can often change as a result of changes in temperature, supply voltage, or even in response to a data signal being applied to the circuit. When a transistor is being used as an amplifier, to achieve the maximum switching speed for that amplifier the bias current of the switching transistor should be maintained at or near IC for peak FT where FT is the transition frequency and IC is the current flowing through the collector of the transistor. It will thus be apparent that if the DC voltage of the transistor changes, this will often cause a change in the bias current, leading to a reduction in the high speed performance of the circuit.
It is therefore an object of the present invention to provide a DC feedback control circuit which can overcome, or at least alleviate the problems caused by a fluctuating DC level.
Accordingly, in a first aspect, the present invention provides a DC feedback control circuit for controlling the DC level of a circuit element having a current electrode of a transistor coupled to the signal output terminal, the DC feedback control circuit comprising an input terminal for coupling to a signal output terminal of the circuit element, a filter coupled to the input terminal, a comparator coupled to the filter and to a reference voltage input for comparing a DC voltage level at the signal output terminal with a reference voltage, a current mirror circuit coupled to the comparator and having an output terminal for coupling to a current electrode of the transistor of the circuit element, wherein the current mirror circuit mirrors the current output of the comparator to the output terminal.
According to a second aspect of the present invention, there is provided a high frequency amplifier comprising an amplifier circuit having an input, at least one transistor having a first current electrode coupled to a first voltage supply rail and a second current electrode coupled to a second voltage supply rail, and a signal output terminal coupled to the first current electrode of the at least one transistor, and a DC feedback control circuit as described above wherein the input terminal is coupled to the signal output terminal and the output terminal is coupled to the second current electrode of the at least one transistor.
In a third aspect, the invention provides a method of compensating for a DC voltage level shift in a circuit using a resistor as a current source and a transistor as a high speed switching device, the method comprising the steps of determining a DC voltage level at an output of the circuit, comparing the DC voltage level with a reference voltage, mirroring a current dependent on the comparison of the DC voltage level with the reference voltage, and applying the mirrored current to the transistor at an emitter thereof to maintain a substantially constant bias current therethrough.
An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawing which shows a circuit diagram of a DC feedback control circuit according to one embodiment of the present invention used to control the DC level of a transimpedance amplifier.